![]() When more and more elements were discovered at the end of the 18 th and the beginning of the 19 th century, there was a growing demand for a clearly laid-out graphic classification of these elements for educational purposes, and also as a guiding tool in the research laboratory. ii) The conviction that chemical periodicity cannot be derived from physics, and that Mendeleev's principle of periodically increasing valence is not the basic point. With the preparations for the 2019-sesquicentenary of the periodic table of elements, two different developments were stimulated at the same time: i) The deeper rational understanding of the physical origin of the chemical periodicity of the basic properties of the elements, in particular of the (maximum) group valence G, and the correspondingly designed Standard Periodic Table (reviewed below). There is presently no need for graphical or other changes, but existing “irregularities” of single atoms or groups of atoms, mainly those of higher values of Z, require dedicated considerations based on the laws of physics which allow to rationalize unexpected deviations from the Aufbau principle and other rules-of-thumb. A graphic representation as introduced in IUPAC's Red Book has served well for education and research in chemistry. The current knowledge of the electronic states of atoms and molecules now calls for a standard form of the periodic table which reflects the large energy gaps between the noble-gas (sp) 8 core shells and the electronic open spdf valence shells, which physically determine the chemical periodicity of rows with lengths increasing in double-steps. The atomic masses (co-determined by the assorted isotopes of an element) were replaced by the atomic number Z. In their and in later more refined periodic tables the periodicity was initially highlighted for values of G from 0 to 8 but long tables with 18 groups were soon required to account for the growing number of elements and their chemical properties. ![]() Whereas while considering sodium and magnesium, these are in the same period.in periods the atomic size decreases due to the nuclear charge on outer electrons.The periodic depictions of the natural system of elements produced by Mendeleyev and his contemporaries were based on atomic mass A and chemical valence G of the elements then already discovered. Due to the small size of the fluorine atom, it is more reactive than chlorine. Fluorine and chlorine belong to the same group. The atomic size increases along with the group. Due to this reason, fluorine is more reactive than chlorine, as fluorine has more tendency to attract electrons than chlorine. Fluorine is the most electronegative element in the periodic table. The elements in group $ 17 $ are halogens, these are considered as non-metals. Thus, the atomic size of $ Mg $ is less than that of $ Na $. In periods, the atomic radius decreases along the period. While considering $ Mg $ and $ Na $, these two elements belong to the same period. Due to the presence of one valence electron, $ Li $ and $ Na $ are considered active metals. ![]() Thus, these elements are known as active metals. These alkali earth metals have only one valence electron in their outermost shell and this one electron can be easily lost. The chemical elements in the first group were known as alkali earth metals. The above periodic table given is the first three periods only. The atomic radius increases along with the groups and decreases along the periods. Hint: The periodic table is the representation of chemical elements arranged in the increasing order of atomic numbers. ![]()
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